To take a blood sample in an intensive care unit, for instance, when the patient has already been provided with an arterial access system, blood is manually aspired by means of an aspiration piston of the infusion set and withdrawn by means of a one-way syringe via the septum of the infusion set. The blood sample is then analysed in an external blood gas analyser or sent to a laboratory for analysis after transfer into a tubule. Withdrawal via the septum presents a certain risk of injury or infection for patient and personnel, and transport to the external blood gas analyser or sending to the laboratory may entail errors due to wrong labelling or confounding of samples.